JP7841857B2 - Method for forming aerobic granules, apparatus for forming aerobic granules - Google Patents

Description

This disclosure relates to a method for forming aerobic granules and an apparatus for forming aerobic granules.

Traditionally, the activated sludge method, which utilizes microbial aggregates called flocs (aerobic biological sludge), has been used for the biological treatment of organic matter-containing wastewater. However, in the activated sludge method, when separating the flocs (aerobic biological sludge) from the treated water in the sedimentation tank, the slow settling rate of the flocs sometimes necessitates a very large surface area in the sedimentation tank. Furthermore, the treatment rate of the activated sludge method depends on the sludge concentration in the biological treatment tank. While increasing the sludge concentration can increase the treatment rate, increasing the sludge concentration to the range of 1500-5000 mg/L or higher can make solid-liquid separation difficult due to bulking in the sedimentation tank, potentially making it impossible to maintain treatment.

On the other hand, anaerobic biological treatment typically utilizes granules, which are granular aggregates of microorganisms. These granules have a very fast settling rate, and because the microorganisms are densely aggregated, a high sludge concentration can be achieved in the treatment tank, enabling high-speed wastewater treatment. However, anaerobic biological treatment has limitations compared to aerobic treatment (activated sludge method), such as being limited to certain types of wastewater and requiring the treatment water temperature to be maintained at around 30-35°C. Furthermore, anaerobic biological treatment alone may result in poor water quality, requiring additional aerobic treatment, such as the activated sludge method, before discharge into rivers or other waterways.

In recent years, it has become clear that by using a semi-batch treatment system that intermittently infuses wastewater into the reaction tank, and by further shortening the sedimentation time of the biological sludge, it is possible to form granulated biological sludge with good settling properties not only for anaerobic biological sludge but also for aerobic biological sludge (see, for example, Patent Documents 1-4). Granulating aerobic biological sludge makes it possible to achieve an average particle size of 0.2 mm or more and a sedimentation velocity of 5 m/h or more. In a semi-batch treatment system, it is common to repeatedly perform four steps in a single biological treatment tank: (1) inflow of wastewater, (2) biological treatment of organic matter, (3) sedimentation of biological sludge, and (4) discharge of treated water.

Furthermore, Patent Document 5 discloses a semi-batch treatment method that repeatedly performs three steps: (1) inflow of wastewater and discharge of treated water, (2) biological treatment of organic matter, and (3) sedimentation of biological sludge.

International Publication No. 2004/024638 Japanese Patent Publication No. 2008-212878 Patent No. 4975541 Patent No. 4804888 Japanese Patent Publication No. 2016-77931

Incidentally, in the past, when organic matter subjected to biological treatment contained a large amount of slow-degrading organic matter, the formation of aerobic granules sometimes did not proceed smoothly.

Therefore, the purpose of this disclosure is to provide a method for forming aerobic granules and an apparatus for forming aerobic granules that can stably form aerobic granules even when organic matter-containing wastewater contains a large amount of slow-degrading organic matter.

This disclosure provides a method for forming aerobic granules using a semi-batch reactor, which forms aerobic granules by performing an operating cycle comprising: an inflow step of introducing organic matter-containing wastewater; a biological treatment step of biologically treating the organic matter in the organic matter-containing wastewater with microbial sludge; a sedimentation step of settling the microbial sludge; and a discharge step of discharging the biologically treated water, wherein the organic matter includes easily decomposable organic matter and slowly decomposable organic matter, and the time of the biological treatment step is adjusted so that the ratio of the MLSS concentration in the semi-batch reactor to the BOD load of the easily decomposable organic matter in the semi-batch reactor is multiplied by [time of the operating cycle / time of the biological treatment step] to be in the range of 0.05 to 0.125 kgBOD/kgMLSS/day, and the sludge retention time in the semi-batch reactor is in the range of 5 to 25 days.

Furthermore, in the method for forming the aerobic granules, it is preferable that the ratio of the BOD concentration of the slow-degrading organic matter in the organic matter-containing wastewater to the total BOD concentration of the organic matter-containing wastewater flowing into the semi-batch reaction vessel is 0.5 or more.

Furthermore, in the method for forming the aerobic granules, it is preferable to provide the biological treatment water outlet of the semi-batch reaction tank above the wastewater inlet, and to allow the organic matter-containing wastewater to flow into the semi-batch reaction tank from the wastewater inlet, thereby discharging the biological treatment water from the biological treatment water outlet.

Furthermore, this disclosure relates to an aerobic granule forming apparatus comprising a semi-batch reactor that forms aerobic granules by performing an operating cycle comprising an inflow step of introducing organic matter-containing wastewater, a biological treatment step of biologically treating the organic matter in the organic matter-containing wastewater with microbial sludge, a sedimentation step of settling the microbial sludge, and a discharge step of discharging the biologically treated water, wherein the organic matter includes easily decomposable organic matter and slowly decomposable organic matter, and the apparatus comprises means for adjusting the time of the biological treatment step such that the ratio of the MLSS concentration in the semi-batch reactor to the BOD load of the easily decomposable organic matter in the semi-batch reactor is multiplied by [time of the operating cycle / time of the biological treatment step] to be in the range of 0.05 to 0.125 kgBOD/kgMLSS/day, and the sludge retention time in the semi-batch reactor is in the range of 5 to 25 days.

Furthermore, this disclosure relates to a wastewater treatment method characterized by supplying aerobic granules formed by the above-described method to a continuous biological treatment tank that continuously receives organic matter-containing wastewater and biologically treats the said organic matter-containing wastewater with biological sludge.

Furthermore, this disclosure provides a wastewater treatment apparatus characterized by comprising a continuous biological treatment tank that continuously receives organic matter-containing wastewater and biologically treats the said organic matter-containing wastewater with biological sludge, and means for supplying aerobic granules formed by the aerobic granule forming apparatus to the continuous biological treatment tank.

According to this disclosure, it is possible to provide a method and apparatus for forming aerobic granules that can stably form aerobic granules even when organic matter-containing wastewater contains a large amount of slow-degrading organic matter.

This is a schematic diagram showing an example of an aerobic granule forming apparatus according to the present disclosure. This is a schematic diagram showing another example of an aerobic granule forming apparatus according to the embodiments of this disclosure. This is a schematic diagram showing another example of an aerobic granule forming apparatus according to the embodiments of this disclosure. This is a schematic diagram showing another example of an aerobic granule forming apparatus according to the embodiments of this disclosure. This is a schematic diagram showing an example of a wastewater treatment apparatus according to the present disclosure. This figure shows the daily changes in SVI and average sludge particle size in the comparative example. This figure shows the daily changes in SVI and average sludge particle size in the example.

The embodiments of this disclosure are described below. These embodiments are examples of implementing this disclosure, and this disclosure is not limited to these embodiments.

<Method and apparatus for forming aerobic granules>
Figure 1 shows a schematic diagram of an example of an aerobic granule forming apparatus according to the present disclosure, and its configuration will be described. The granule forming apparatus 1 includes a semi-batch reaction tank 10. In the granule forming apparatus 1, a wastewater supply pipe 28 is connected to the wastewater inlet of the semi-batch reaction tank 10 via a wastewater inlet pump 12. A biological treatment water pipe 30 is connected to the biological treatment water outlet 16 of the semi-batch reaction tank 10 via a biological treatment water discharge valve 18, and a sludge extraction pipe 32 is connected to the sludge extraction port 22 via a sludge extraction pump 24. An aeration device 26 connected to an aeration pump 14 is installed in the lower part of the inside of the semi-batch reaction tank 10.

The granule forming apparatus 1 is equipped with a control device 20. The control device 20 consists of a microcomputer comprising, for example, a CPU for calculating programs, ROM and RAM for storing programs and calculation results, and electronic circuits. It reads a predetermined program stored in the ROM, executes the program, and controls the operation of the granule forming apparatus 1. The control device is electrically connected to, for example, the wastewater inflow pump 12, the biological treatment water discharge valve 18, the sludge extraction pump 24, and the aeration pump 14, and controls the operation and stopping of the pumps, the opening and closing of the valves, etc.

The granule forming apparatus 1 is operated, for example, in the following cycle:

(1) Inflow Process: The wastewater inflow pump 12 is activated, and a predetermined amount of organic matter-containing wastewater flows into the semi-batch reaction vessel 10 through the wastewater supply pipe 28.

(2) Biological Treatment Process: When the wastewater inlet pump 12 stops, oxygen-containing gas such as air is supplied from the aeration pump 14 to the semi-batch reaction tank 10 through the aeration device 26. In the semi-batch reaction tank 10, organic matter and other substances to be treated in the organic matter-containing wastewater are biologically treated by microbial sludge. The biological reaction is not limited to aerobic reactions; it is also possible to perform an anaerobic reaction by stirring without supplying air, or to combine aerobic and anaerobic reactions. An anaerobic state refers to a state where dissolved oxygen is absent, but oxygen derived from nitrite or nitrate is present. For example, as shown in Figure 2, a stirring device consisting of a motor 34, a stirring blade 36, and a shaft connecting the motor 34 and the stirring blade 36 can be installed in the semi-batch reaction tank 10, and stirring can be performed by stopping the aeration pump 14 and using the stirring device. Note that the stirring device is not limited to the above configuration.

(3) Settlement process: The aeration pump 14 is stopped, and the tank is left to stand for a predetermined time to allow the sludge in the semi-batch reaction tank 10 to settle.

(4) Discharge Process: By opening the biologically treated water discharge valve 18, the supernatant water obtained in the sedimentation process is discharged as biologically treated water from the biologically treated water discharge port 16 through the biologically treated water piping 30. In this case, the biologically treated water may be discharged using a pump instead of the biologically treated water discharge valve 18.

By repeating the operation cycle consisting of the above steps (1) to (4), aerobic granules (hereinafter simply referred to as granules), which are aggregates of microorganisms that have densely gathered into granular form, are formed.

The granules formed in the semi-batch reaction tank 10 are sludge that has undergone self-granulation. For example, the sludge has an average particle size of 0.2 mm or more, or a sedimentation index (SVI5) of 80 mL/g or less. In this embodiment, whether or not granules have formed is determined, for example, by measuring the SVI, the sedimentation index of the sludge. Specifically, it is possible to determine that granules have formed when the SVI5 value, measured periodically through sedimentation tests of the sludge in the semi-batch reaction tank 10, falls below a predetermined value (for example, 80 mL/g or less). Alternatively, it is possible to determine that granules have formed when the particle size distribution of the sludge in the semi-batch reaction tank 10 is measured and the average particle size exceeds a predetermined value (for example, 0.2 mm or more). (Note that a lower SVI value and a larger average particle size indicate better granules.)

Incidentally, the BOD load of the semi-batch reactor 10 is determined by the product of the BOD concentration and the amount of organic matter-containing wastewater flowing into the semi-batch reactor 10. The BOD concentration is a value measured from the amount of oxygen consumed when microorganisms decompose organic matter over five days. However, the organic matter in the organic matter-containing wastewater includes slow-degrading organic matter, which takes approximately several tens of hours to several days for biodegradation by microorganisms, and easily degradable organic matter, which takes approximately several hours to several tens of hours for biodegradation by microorganisms. Therefore, the BOD concentration can be classified into the total BOD concentration, which corresponds to the amount of oxygen consumed by microorganisms when decomposing organic matter including slow-degrading and easily degradable organic matter; the BOD concentration of slow-degrading organic matter, which corresponds to the amount of oxygen consumed by microorganisms when decomposing slow-degrading organic matter; and the BOD concentration of easily degradable organic matter, which corresponds to the amount of oxygen consumed by microorganisms when decomposing easily degradable organic matter. The total BOD concentration is the sum of the BOD concentrations of slow-degrading organic matter and the BOD concentrations of easily degrading organic matter.

Therefore, the BOD load of the semi-batch reactor 10 can be classified into three categories: the total BOD load based on the total BOD concentration (total BOD concentration × amount of wastewater containing organic matter), the BOD load of slow-degrading organic matter based on the BOD concentration of slow-degrading organic matter (BOD concentration of slow-degrading organic matter × amount of wastewater containing organic matter), and the BOD load of easily degradable organic matter based on the BOD concentration of easily degradable organic matter (BOD concentration of easily degradable organic matter × amount of wastewater containing organic matter). The total BOD load is the sum of the BOD load of slow-degrading organic matter and the BOD load of easily degradable organic matter.

Here, for stable granule formation, it is important to control the ratio between the satiety state, when the organic matter concentration in the organic matter-containing wastewater flowing into the semi-batch reactor 10 is high, and the starvation state, when the organic matter decomposition by microbial sludge progresses and the organic matter concentration in the organic matter-containing wastewater is low. This relationship between the satiety state and the starvation state can be indirectly controlled by using the ratio of the MLSS concentration in the semi-batch reactor 10 to the BOD load. Furthermore, since processes other than the biological treatment process do not significantly contribute to the biological reaction, it is possible to control the satiety time/starvation time ratio more precisely by evaluating the value obtained by multiplying the ratio of MLSS concentration to BOD load by [operating cycle time / biological treatment process time]. Here, "operating cycle time" refers to the total time of the above-mentioned (1) inflow process, (2) biological treatment process, (3) sedimentation process, and (4) discharge process (in the case of the configurations shown in Figures 3 and 4 below, it refers to the total time of (1) inflow/discharge process, (2) biological treatment process, and (3) sedimentation process).

However, if the wastewater containing organic matter contains a large amount of slow-degrading organic matter, and the ratio of MLSS concentration to total BOD load is used as the ratio of MLSS concentration to BOD load to determine the time of the biological treatment process, the balance between the satiety state and the starvation state in the operating cycle will be disrupted, making it difficult to form stable granules. Therefore, after diligent research by the inventors, it was found that when the wastewater containing organic matter contains a large amount of slow-degrading organic matter, it is important to determine the time of the biological treatment process by using the ratio of MLSS concentration to BOD load of easily degradable organic matter to ensure stable granule formation. Specifically, the inventors discovered that stable granule formation is possible by adjusting the time of the biological treatment process so that the ratio of the MLSS concentration in the semi-batch reactor 10 to the BOD load of easily degradable organic matter (BOD load of easily degradable organic matter / MLSS) multiplied by [operating cycle time / biological treatment process time] (hereinafter sometimes referred to as "value A") is in the range of 0.05 to 0.25 kgBOD/kgMLSS/day.

The "A value" is preferably in the range of 0.05 to 0.25 kgBOD/kgMLSS/d, and more preferably in the range of 0.075 to 0.2 kgBOD/kgMLSS/d. If this value is less than 0.05 kgBOD/kgMLSS/d, appropriate satiety and starvation states cannot be formed, making stable granule formation difficult. Conversely, if this value is greater than 0.25 kgBOD/kgMLSS/d, the starvation period will be too short, making stable granule formation difficult.

The following describes a method for calculating the BOD load of easily decomposable organic matter. The following calculation method is illustrative and not limited to it.

<Example 1 of calculating the BOD load of easily decomposable organic matter>
The change in the oxygen consumption rate of microbial sludge in organic matter-containing wastewater over time is measured. The oxygen consumption rate is determined by the known OUR (Oxygen Uptake Rate) test. The OUR test is performed, for example, by mixing wastewater and microbial sludge and reacting them in batches, and measuring the oxygen consumption rate of the microbial sludge over time. It is preferable that the microbial sludge used in the OUR test is well acclimated to the wastewater under test. When well acclimated microbial sludge is used, the oxygen consumption rate is highest immediately after the start of the test and then gradually decreases. This is because the decomposition rate of easily decomposable organic matter in organic matter-containing wastewater is fast, so the amount of easily decomposable organic matter decreases over time, and the proportion of slowly decomposable organic matter increases.

Then, by dividing the oxygen consumption rate measured at any given time by the sludge concentration of the sludge under test, the change in the oxygen consumption rate per unit of microbial sludge over time is determined. The time during which the oxygen consumption rate per unit of microbial sludge is maintained at, for example, 0.4 _kgO₂ /kgMLVSS/d or higher is judged to indicate the presence of readily decomposable organic matter, and the cumulative oxygen consumption used up to that point is estimated to be the BOD concentration of the readily decomposable organic matter. The BOD load of readily decomposable organic matter is calculated by multiplying the estimated BOD concentration of readily decomposable organic matter by the amount of organic matter-containing wastewater introduced into the semi-batch reaction tank.

<Example 2 of calculating the BOD load of easily decomposable organic matter>
As a typical example of slow-degrading organic matter, solid organic matter is a good example. Therefore, if wastewater contains a high concentration of organic suspended solids (SS) components, the BOD concentration of the slow-degrading organic matter may be calculated using a formula (which may also be a map, table, etc.) that specifies the relationship between the BOD concentration of the slow-degrading organic matter and the organic SS components, which has been determined in advance. Then, the BOD concentration of easily degradable organic matter may be determined by subtracting the BOD concentration of the slow-degrading organic matter, calculated from the total BOD concentration measured separately, and the BOD load of easily degradable organic matter may be calculated. In this case, it is also possible to determine the concentration of slow-degrading organic matter in real time by providing a means for measuring the organic SS components in the wastewater (SS meter or turbidimeter, etc.) and monitoring the concentration. This calculation method is suitable for wastewater with an SS concentration of 100 mg/L or more and is particularly effective when dealing with raw sewage (influent sewage that has not undergone pretreatment such as sedimentation) as the target wastewater.

<Example 3 of calculating the BOD load of easily decomposable organic matter>
In the case of wastewater where the ratio of slow-degrading organic matter to easily degradable organic matter does not fluctuate significantly, the BOD concentration of easily degradable organic matter (or the BOD concentration of slow-degrading organic matter) can be calculated from an equation (which may also be a map, table, etc.) that defines the relationship between the BOD concentration of easily degradable organic matter (or the BOD concentration of slow-degrading organic matter) and the COD concentration and TOC concentration, and the BOD load of easily degradable organic matter can be calculated. In this case, the BOD concentration of easily degradable organic matter (or the BOD concentration of slow-degrading organic matter) can be determined in real time by monitoring the COD concentration and TOC concentration of the organic matter-containing wastewater flowing into the semi-batch reactor.

In the semi-batch reaction tank 10, the sludge retention time (SRT) is preferably in the range of 5 to 25 days, and more preferably in the range of 10 to 15 days, from the standpoint of stable granule formation. For example, the sludge extraction pump 24 shown in Figures 1 and 2 is operated so that the SRT is in the range of 5 to 25 days, and sludge is extracted from the sludge extraction port 22 through the sludge extraction piping 32. Note that if the "A value" is below 0.05, the proportion of microorganisms capable of growth is small, and the amount of sludge extracted cannot be increased, making it difficult to extend the SRT beyond 30 days; approximately 25 days is the limit.

SRT is expressed by the following formula:
SRT[d] = Amount of sludge present in the tank [kg] / Amount of sludge discharged from the system per day [kg/d]
Furthermore, the MLSS concentration in the semi-batch reaction vessel 10 is preferably in the range of 1,500 to 10,000 mg/L, and more preferably in the range of 3,000 to 8,000 mg/L, although this depends on the total BOD load, in terms of stable granule formation.
Furthermore, the ratio of the MLSS concentration to the total BOD load including slow-degrading and easily degrading organic matter in the semi-batch reaction tank 10, multiplied by [the time of the operation cycle / the time of the biological treatment process], is preferably in the range of 1.0 kg BOD/kg MLSS/d or less, and more preferably in the range of 0.5 kg BOD/kg MLSS/d or less. If it is 1.0 kg BOD/kg MLSS/d or more, undegraded BOD components may accumulate in the sludge in the reaction tank, worsening the settling properties, or granule formation and maintenance may become difficult due to the appearance of filamentous fungi that cause poor sludge settling.

The organic wastewater treated by the granule formation method according to this embodiment includes organic wastewater containing biodegradable organic matter, such as wastewater from food processing plants, chemical plants, semiconductor plants, machinery factories, sewage, and human waste. Furthermore, if the wastewater contains organic matter that is difficult to biodegrade, it can be treated by pre-treating it with physicochemical treatments such as ozone treatment or Fenton treatment to convert it into biodegradable components. While the granule formation method according to this embodiment targets various BOD components, oil and grease may adhere to sludge and granules and have adverse effects. Therefore, it is preferable to remove them to approximately 150 mg/L or less before introducing them into the semi-batch reaction tank 10 using existing methods such as flotation separation, coagulation and pressurized flotation, or adsorption.

The pH in the semi-batch reaction vessel 10 is preferably set within a range suitable for general microorganisms, for example, preferably in the range of 6 to 9, and more preferably in the range of 6.5 to 7.5. If the pH value falls outside this range, it is preferable to control the pH by adding acid, alkali, etc.

The dissolved oxygen (DO) in the semi-batch reaction vessel 10 is preferably 0.5 mg/L or more, and more preferably 1 mg/L or more, under aerobic conditions.

In order to promote the granulation of microbial sludge, it is preferable to add ions that form hydroxides, such as ^Fe²⁺ , ^Fe³⁺ , ^Ca²⁺ , and ^Mg²⁺ , to the organic matter-containing wastewater in the semi-batch reaction tank 10 or to the organic matter-containing wastewater before it is introduced into the semi-batch reaction tank 10. Ordinary organic matter-containing wastewater contains fine particles that serve as nuclei for granules, but the addition of the above ions makes it possible to further promote granule nucleation.

Another example of a granule forming apparatus according to this embodiment is shown in Figure 3. In the granule forming apparatus 1 of Figure 3, a wastewater supply pipe 28 is connected to a wastewater inlet 40 at the bottom of a semi-batch reaction tank 10 via a wastewater inlet pump 12 and a wastewater inlet valve 38. A wastewater discharge section 42 is connected to the wastewater inlet 40 and is installed in the lower part of the inside of the semi-batch reaction tank 10. The biological treated water outlet 16 of the semi-batch reaction tank 10 is provided above the wastewater inlet 40, and a biological treated water pipe 30 is connected to the biological treated water outlet 16 via a biological treated water discharge valve 18. Although the biological treated water outlet 16 is provided above the wastewater inlet 40, it is preferable that it be provided as far away from the wastewater inlet 40 as possible in order to prevent short circuits of the incoming organic matter-containing wastewater and to form granules more efficiently, and it is more preferable that it be provided at the water level during the settling process. The control device 20 is electrically connected, for example, to the wastewater inlet pump 12, wastewater inlet valve 38, biological treated water discharge valve 18, sludge extraction pump 24, aeration pump 14, and the motor 34 of the agitator. Otherwise, the configuration is the same as that of the granule forming apparatus 1 shown in Figure 2.

In the granule forming apparatus 1 shown in Figure 3, during the (4) discharge process, the wastewater inlet valve 38 is opened and the wastewater inlet pump 12 is activated. The organic matter-containing wastewater is then introduced from the wastewater inlet 40 through the wastewater supply pipe 28 to the wastewater discharge section 42 and into the semi-batch reaction tank 10. The biologically treated water is then discharged from the biologically treated water outlet 16 through the biologically treated water pipe 30.

As described above, in the granule forming apparatus 1 shown in Figure 3, granules are formed by repeating the following steps: (1) inflow/discharge process, (2) biological treatment process, and (3) sedimentation process. This repeating of steps (1) to (3) constitutes one form of an operating cycle comprising an inflow process, a biological treatment process, a sedimentation process, and a discharge process.

In the granule formation apparatus 1 shown in Figure 3, organic matter-containing wastewater is introduced into the semi-batch reaction tank 10, and the biologically treated water is discharged from the biologically treated water outlet 16. Therefore, granules with relatively small particle sizes are discharged along with the biologically treated water, and steps (1) to (3) are repeated for granules with relatively large particle sizes. As a result, granules can be formed more efficiently.

The wastewater inflow rate in the inflow/discharge process is preferably in the range of 10% to 100%. The wastewater inflow rate is the ratio of the amount of treated water inflow in one operating cycle to the effective volume in the semi-batch reactor 10. While a higher inflow rate of treated water is desirable to increase the concentration of the target substance remaining in the semi-batch reactor 10, a higher wastewater inflow rate raises concerns about water deterioration due to short circuits. Therefore, considering these factors, a wastewater inflow rate of 20% to 80% is more preferable. However, if a treatment device such as an activated sludge tank is installed downstream of the semi-batch reactor 10, and the water quality of the final treated water after the downstream treatment device does not deteriorate, there are no particular restrictions on the wastewater inflow rate; for example, it can exceed 100%. When the wastewater inflow rate exceeds 100%, it is preferable to limit the upper limit of the wastewater inflow rate to 200% or less in order to suppress a decrease in the number of operating cycles.

The time for the inflow/discharge process is determined, for example, according to the inflow rate of wastewater and the flow rate of water to be treated into the semi-batch reaction tank 10. However, if the water surface load of the semi-batch reaction tank 10, which is the value obtained by dividing the flow rate of wastewater into the semi-batch reaction tank 10 by the horizontal cross-sectional area of the semi-batch reaction tank 10, is set high, it becomes possible to selectively discharge the lighter sludge fraction from the sludge outside the system and leave the highly sedimentable sludge fraction in the tank. This promotes the formation of highly sedimentable biological sludge. However, during the start-up period, when the sludge's sedimentability is not high, there is a concern that sludge in the tank may flow out, leading to a deterioration of the biological treatment function. On the other hand, if the water surface load of the semi-batch reaction tank 10 is set low, the selective effect of the sludge decreases, and if the wastewater inflow rate is further increased, the inflow/discharge process time becomes longer, raising concerns that it may become difficult to form highly sedimentable sludge. Considering the above circumstances, it is preferable that the water surface load on the semi-batch reaction tank 10 be between 0.5 m/h and 20 m/h, and preferably within the range of 1 m/h to 10 m/h. Furthermore, if the settling rate of the biological sludge in the tank improves, making it possible to set a higher water surface load on the semi-batch reaction tank 10, the water surface load of the semi-batch reaction tank 10 can be increased according to the settling rate of the biological sludge, and the inflow/outflow process time can be shortened according to the water surface load and the inflow rate of the treated water.

Another example of an aerobic granule forming apparatus according to this embodiment is shown in Figure 4. In the granule forming apparatus 1 of Figure 4, a wastewater supply pipe 28 is connected to a wastewater inlet 40 at the bottom of a semi-batch reaction tank 10 via a wastewater inlet pump 12 and a wastewater inlet valve 38. A wastewater discharge section 42 is connected to the wastewater inlet 40 and is installed in the lower part of the inside of the semi-batch reaction tank 10. The biological treated water outlet 16 of the semi-batch reaction tank 10 is provided above the wastewater inlet 40, and a biological treated water pipe 30 is connected to the biological treated water outlet 16 via a biological treated water discharge valve 18. Although the biological treated water outlet 16 is provided above the wastewater inlet 40, it is preferable that it be provided as far away from the wastewater inlet 40 as possible in order to prevent short circuits of incoming organic matter-containing wastewater and to form granules more efficiently, and it is more preferable that it be provided at the water level during the settling process. The control device 20 is electrically connected, for example, to the wastewater inflow pump 12, the wastewater inflow valve 38, the biological treated water discharge valve 18, the sludge extraction pump 24, and the aeration pump 14. Otherwise, the configuration is the same as that of the granule forming apparatus 1 shown in Figure 1.

In the granule forming apparatus 1 shown in Figure 4, during the (4) discharge process, the wastewater inlet valve 38 is opened and the wastewater inlet pump 12 is activated. This allows organic matter-containing wastewater to flow from the wastewater inlet 40 through the wastewater supply pipe 28 to the wastewater discharge section 42 and into the semi-batch reaction tank 10. The biologically treated water is then discharged from the biologically treated water outlet 16 through the biologically treated water pipe 30. The operation and stopping of the wastewater inlet pump 12, sludge extraction pump 24, and aeration pump 14, as well as the opening and closing of the wastewater inlet valve 38 and the biologically treated water discharge valve 18, may be controlled by the control device 20.

Thus, in the granule forming apparatus 1 shown in Figure 4, granules are formed by repeating the following steps: (1) inflow/discharge process, (2) biological treatment process, and (3) sedimentation process.

<Wastewater treatment methods and wastewater treatment equipment>
The wastewater treatment apparatus according to this embodiment includes a continuous biological treatment tank that continuously receives organic wastewater containing organic matter and biologically treats the organic wastewater containing organic matter with biological sludge. In the wastewater treatment method and wastewater treatment apparatus according to this embodiment, granules formed by the above-described aerobic granule formation method are supplied to the continuous biological treatment tank that continuously receives organic wastewater containing organic matter and biologically treats the organic wastewater containing organic matter with biological sludge.

Figure 5 shows a schematic configuration of an example of a wastewater treatment apparatus according to this embodiment. The wastewater treatment apparatus 3 comprises a wastewater storage tank 50, a semi-batch reaction tank 10, a continuous biological treatment tank 52, and a solid-liquid separation device 54.

In the wastewater treatment device 3, the outlet of the wastewater storage tank 50 and the wastewater inlet of the continuous biological treatment tank 52 are connected by a wastewater supply pipe 66 via a pump 56 and a valve 58. The outlet of the continuous biological treatment tank 52 and the inlet of the solid-liquid separator 54 are connected by a pipe 70. A treated water pipe 72 is connected to the treated water outlet of the solid-liquid separator 54. A sludge discharge pipe 74 is connected to the sludge outlet of the solid-liquid separator 54 via a valve 62, and the upstream side of the valve 62 of the sludge discharge pipe 74 and the return sludge inlet of the continuous biological treatment tank 52 are connected by a sludge return pipe 76 via a pump 64. The space between the pump 56 and the valve 58 of the wastewater supply pipe 66 and the wastewater inlet of the semi-batch reaction tank 10 are connected by a wastewater inlet valve 38 via a wastewater supply pipe 28. The biological treatment water outlet of the semi-batch reaction tank 10 and the biological treatment water inlet of the continuous biological treatment tank 52 are connected by a biological treatment water discharge valve 18 and a biological treatment water piping 30. The sludge outlet of the semi-batch reaction tank 10 and the sludge inlet of the continuous biological treatment tank 52 are connected by a pump 60 and a sludge piping 68.

The continuous biological treatment tank 52 is equipped with, for example, a stirring device, an aeration pump, and an aeration device connected to the aeration pump. The stirring device agitates the liquid inside the tank, and oxygen-containing gases such as air supplied from the aeration pump are supplied to the tank through the aeration device.

The solid-liquid separation device 54 is a separation device for separating biological sludge from treated water containing biological sludge into biological sludge and treated water. Examples of such separation devices include sedimentation separation, pressurized flotation, filtration, and membrane separation.

In the wastewater treatment device 3, first, valve 58 is opened, pump 56 is activated, and organic matter-containing wastewater from the wastewater storage tank 50 is supplied to the continuous biological treatment tank 52 through the wastewater supply pipe 66. In the continuous biological treatment tank 52, biological treatment of the wastewater with biological sludge is carried out under aerobic conditions (continuous biological treatment process). The treated water treated in the continuous biological treatment tank 52 is supplied from the outlet of the continuous biological treatment tank 52 through pipe 70 to the solid-liquid separator 54. In the solid-liquid separator 54, biological sludge is separated from the treated water (solid-liquid separation process). The treated water that has undergone solid-liquid separation is discharged from the solid-liquid separator 54 through the treated water pipe 72. The solid-liquid separated biological sludge is discharged from the system through the sludge discharge pipe 74 by opening valve 62. Pump 64 may be activated to return at least a portion of the solid-liquid separated biological sludge to the continuous biological treatment tank 52 through the sludge return pipe 76.

When operating the semi-batch reaction tank 10, the wastewater inlet valve 38 is opened, and at least a portion of the organic matter-containing wastewater in the wastewater storage tank 50 is supplied to the semi-batch reaction tank 10 through the wastewater supply pipe 28. In the semi-batch reaction tank 10, the operation cycle of (1) inlet process, (2) biological treatment process, (3) sedimentation process, and (4) discharge process (or the operation cycle of (1) inlet process/discharge process, (2) biological treatment process, and (3) sedimentation process) is repeated to form granules. The pump 60 is then operated, and the formed granules are supplied to the continuous biological treatment tank 52 through the sludge pipe 68.

In the continuous biological treatment tank 52 shown in Figure 5, a standard activated sludge process for treating organic matter was used as an example. However, the explanation is not limited to this. For example, the apparatus may also perform biological treatment using nutrient removal systems such as A2O (Anaerobic-Anoxic-Oxic Process) or AO (Anaerobic-Oxic Process) (systems that install an oxygen-free treatment tank or an anaerobic treatment tank), oxidation ditch methods, or step-inflow multi-stage activated sludge processes. Furthermore, the apparatus may perform biological treatment in the presence of carriers such as polyurethane, plastic, or resin.

In the wastewater treatment device 3 shown in Figure 5, a configuration including a solid-liquid separator 54 was described as an example, but it is not necessarily required to include the solid-liquid separator 54. However, in terms of improving the wastewater treatment efficiency by circulating the granules, it is preferable for the wastewater treatment device 3 to include a solid-liquid separator 54 that separates biological sludge from the treated water discharged from the continuous biological treatment tank 52, and a sludge return pipe 76 that returns the biological sludge discharged from the solid-liquid separator 54 to the continuous biological treatment tank 52.

The present disclosure will be described in more detail below with reference to examples and comparative examples, but this disclosure is not limited to the following examples.

A water flow test was conducted using a semi-batch reactor with an effective reaction vessel volume of 33 L (125 mm x 438 mm x effective water depth of 600 mm). Granulation was evaluated using the values of SVI5 and SVI30 as indicators. SVI is an index of the settling properties of biological sludge and is determined by the following method. First, 1 L of sludge is placed in a 1 L graduated cylinder and gently stirred to make the sludge concentration as uniform as possible, and then the sludge interface is measured after standing for 5 minutes. The volume percentage (%) occupied by the sludge in the graduated cylinder is then calculated. Next, the MLSS (mg/L) of the sludge is measured. These values are then applied to the following formula to calculate SVI5. A smaller SVI5 value indicates that the sludge has higher settling properties.
SVI5 (mL/g) = Volume percentage occupied by sludge × 10,000 / MLSS
(Note that when calculating SVI30, the 5-minute standing period should be changed to 30 minutes.)

The wastewater used was raw sewage flowing into a sewage treatment plant, pre-treated with a 2 mm mesh screen without sedimentation. Table 1 shows the total BOD concentration, easily degradable BOD concentration, and slowly degradable BOD concentration of the raw sewage during the test period. The ratio of slowly degradable BOD concentration to total BOD concentration was 0.5 or higher.

The operating cycle for the semi-batch reactor was carried out as follows.
(1) Inflow/Outflow Process: Over a period of 50 minutes, wastewater was introduced into a semi-batch reaction vessel, and the supernatant water was discharged as treated water. The wastewater inflow rate was 100%.
(2) Biological treatment process: The time for the biological treatment process was set so that the value obtained by multiplying the ratio of MLSS concentration to the BOD load of easily decomposable organic matter in the semi-batch reactor by [operating cycle time / biological treatment process time] (A value in the chemical formula) was the value in Table 2. During the set time, air was supplied from the aeration device installed at the bottom of the semi-batch reactor, and the biological treatment process was carried out.
(3) Settlement process: The supply of air from the aeration device was stopped and the mixture was left to stand for 15 to 30 minutes to allow the sludge in the semi-batch reaction tank to settle.
The above operating cycles (1) to (3) were repeated as one cycle.

The value (A value) obtained by multiplying the ratio of MLSS concentration to the BOD load of easily degradable organic matter in a semi-batch reactor by [operating cycle time / biological treatment process time] can be calculated, for example, as follows.
A=(((B-C)/1000×(H×D/100×G))/(I/1000×H))
× (F/E)
Here,
B = Easily degradable BOD concentration in wastewater [mg/L]
C = Easily degradable BOD concentration after treatment [mg/L]
D = Percentage of wastewater introduced per cycle relative to the effective volume of the reactor [%]
E = Biological processing time per cycle [minutes]
F = Total process time for one cycle [minutes]
G = Number of cycles per day [cycles/day]
H = Effective volume of the reaction vessel [ ^m³ ]
I=MLSS [mg/L]

<Conditions for the biological treatment process>

Figure 6 shows the daily changes in SVI and average sludge particle size under conditions 1-2 (comparative examples) in Table 2, and Figure 7 shows the daily changes in SVI and average sludge particle size under conditions 3-4 (examples) in Table 2.

Under Condition 1, the system was operated so that the MLSS was in the range of 3000-4000 mg/L, and the biological treatment process time was set so that the A value was less than 0.04-0.05 kg BOD/kg MLSS/day. By the 20th day after the start of water flow, SVI30 decreased to approximately 80 mL/g, and SVI5 decreased to 170 mg/L. The particle size of the microbial sludge also increased, with an average particle size of 200 μm. However, after the 20th day, the decrease in SVI and the increase in microbial sludge particle size stagnated.

Under Condition 2, the system was operated so that the MLSS was in the range of 5000-6000 mg/L, and the biological treatment process time was set so that the A value was less than 0.02-0.05 kg BOD/kg MLSS/day. An increase in SVI was observed from around 40 days after the start of water flow. Under Condition 2, the particle size of the microbial sludge hardly changed.

Under condition 3, the system was operated to maintain an MLSS level of approximately 3500 mg/L, and the biological treatment process time was set to achieve an A value of 0.05–0.1 kg BOD/kg MLSS/day. As a result, SVI5 decreased to approximately 100 mL/g. Furthermore, the particle size of the microbial sludge increased, with an average particle size of 300 μm.

Under condition 4, the system was operated so that the MLSS was approximately 4000-5000 mg/L, and the biological treatment process time was set so that the A value was 0.075-0.125 kg BOD/kg MLSS/day. As a result, SVI5 decreased to approximately 40 mL/g, and SVI30 decreased to approximately 30 mL/g. Furthermore, the particle size of the microbial sludge increased, with an average particle size of 350 μm.

1. Granule forming apparatus, 3. Wastewater treatment apparatus, 10. Semi-batch reaction tank, 12. Wastewater inlet pump, 14. Aeration pump, 16. Biologically treated water outlet, 18. Biologically treated water outlet valve, 20. Control device, 22. Sludge extraction port, 24. Sludge extraction pump, 26. Aeration apparatus, 28, 66. Wastewater supply piping, 30. Biologically treated water piping, 32. Sludge extraction piping, 34. Motor, 36. Agitator blade, 38. Wastewater inlet valve, 40. Wastewater inlet, 42. Wastewater discharge section, 50. Wastewater storage tank, 52. Continuous biological treatment tank, 54. Solid-liquid separation apparatus, 56, 60, 64. Pumps, 58, 62. Valves, 68. Sludge piping, 70. Piping, 72. Treated water piping, 74. Sludge discharge piping, 76. Sludge return piping.

Claims (4)

A method for forming aerobic granules using a semi-batch reactor, comprising an operation cycle comprising: an inflow step of introducing organic matter-containing wastewater; a biological treatment step of biologically treating the organic matter in the organic matter-containing wastewater with microbial sludge; a sedimentation step of settling the microbial sludge; and a discharge step of discharging the biologically treated water, wherein aerobic granules are formed by performing this operation cycle.
The aforementioned organic matter includes easily decomposable organic matter and slowly decomposable organic matter.
The time of the biological treatment process is adjusted so that the ratio of the MLSS concentration in the semi-batch reactor to the BOD load of the easily decomposable organic matter in the semi-batch reactor, multiplied by [the time of the operation cycle / the time of the biological treatment process], is in the range of 0.05 to 0.125 kgBOD/kgMLSS/day.
A method for forming aerobic granules, characterized in that the sludge retention time in the aforementioned semi-batch reaction tank is set to a range of 5 to 25 days. The method for forming aerobic granules according to claim 1, characterized in that the ratio of the BOD concentration of the slow-degrading organic matter in the organic matter-containing wastewater to the total BOD concentration of the organic matter-containing wastewater flowing into the semi-batch reaction vessel is 0.5 or more. The method for forming aerobic granules according to claim 1 or 2, characterized in that the biological treatment water outlet of the semi-batch reaction tank is provided above the wastewater inlet, and the organic matter-containing wastewater is allowed to flow into the semi-batch reaction tank from the wastewater inlet, thereby discharging the biological treatment water from the biological treatment water outlet. An aerobic granule forming apparatus comprising a semi-batch reactor that forms aerobic granules by performing an operating cycle comprising: an inflow step of introducing wastewater containing organic matter; a biological treatment step of biologically treating the organic matter in the wastewater containing organic matter with microbial sludge; a sedimentation step of settling the microbial sludge; and a discharge step of discharging the biologically treated water.
The aforementioned organic matter includes easily decomposable organic matter and slowly decomposable organic matter.
The system includes means for adjusting the time of the biological treatment process such that the ratio of the MLSS concentration in the semi-batch reactor to the BOD load of the easily decomposable organic matter in the semi-batch reactor is multiplied by [the time of the operation cycle / the time of the biological treatment process] to be in the range of 0.05 to 0.125 kgBOD/kgMLSS/day.
An aerobic granule forming apparatus characterized in that the sludge retention time in the aforementioned semi-batch reaction tank is in the range of 5 to 25 days.